basis for QDL in 2004. Although it has
attracted far less academic and industrial attention than QKD, it has developed over the past decade into a family
of theoretical encryption techniques.

“QDL shows a phenomenon that
is unique to quantum information
theory; it is not possible in classical
information theory. In application,
we have demonstrated the QDL protocol is able to encrypt a message
and send it over tens of kilometers of
fiber,” Liu claims.

Although it is much younger and
faces a number of challenges, the
attraction of QDL over QKD is that
it is potentially far more efficient in
terms of how much information can
be encrypted for each bit of key than
any system that relies on classical
communication.

To provide provably secure communication, protocols in use today need
to obey a theory developed by Claude
Shannon in the 1940s. The encryption
key, which must be generated randomly, needs to be the same length or
greater than the information content
of the message itself. Shannon’s theory provided support for the one-time
pad developed in the late 19th century,
in which sender and receiver agree to
use a common key—originally taking
the form of characters written on a pad
of paper—only once. Once the message had been received and decoded,
the key was to be discarded.

QKD provides the means for two
parties, Alice and Bob, to agree on a
secret key without risk of it being obtained by an eavesdropper. The protocol takes advantage of the way in which
an attempt to determine one part of
the quantum state of a particle disturbs the others. It makes it impossible
to completely determine the quantum
state of a photon or particle and, as a
result, copy it.

Under QKD, when taking measurements of a sequence of photons
they exchange, Alice and Bob agree
to randomly swap between two different types of measurement of the
quantum state and then compare the
results. Eve can intercept the photon,
perform her own measurements,
and attempt to copy the photon and
pass it on to Bob. They cannot, however, determine the state of the other
property, and the new photon will be

forwarded with a state that probably
does not match Alice’s original.

Without an eavesdropper like Eve
present, Alice and Bob’s measurements
will match approximately half the time
because of their random switching between properties. With an eavesdropper
present, the error rate rises significantly, because of the 50% probability for
each photon that the eavesdropper has
picked the wrong measurement to perform. But if enough of Alice’s and Bob’s
measurements agree, the received pattern becomes a shared private key that
can be used to encrypt messages on another channel, which can use traditional classical coding techniques.

One problem for QKD is the limit on
communication speed caused by the
nature of the protocol itself, combined
with the effects of noise and interference in the quantum channel. Stefano
Pirandola, a researcher at the University of York, says QKD protocols based
on encoding pairs of properties into
‘qubits’ tend to deliver very low key-update rates. One way to boost the update rate is to use continuous-variable
properties such as the quadrature operators of the coherent light transmissions from lasers. These quadrature
operators “play the same role that position and momentum play for a particle
such as an electron,” he says.

The need to use lengthy keys for
message delivery still leaves QKD-based systems facing a potential bottleneck. QDL can harness the difficulties eavesdroppers have in intercepting
quantum channels to send the data
bits themselves and use exponentially
shorter keys than those needed for
Shannon’s one-time pad system.

To employ QDL, Alice and Bob first
agree on a shared key, which could be

generated using QKD. That key selects
a set of codewords that determine the
sequence of properties to be measured
and their contents. Each codeword calls
for a different sequence of measurements on the quantum states. As with
QKD, Eve can only access a fraction of
the complete message, even with access to unlimited computing power.

“I think QDL is an interesting approach that relies on the realistic assumption that today, an eavesdropper
cannot do everything and can only access quantum memories with limited
lifetimes,” says Pirandola.

Liu explains, “We performed two
experiments using our setup. The first
was to show the original data-locking
idea. The protocol locks half of the
message using a 1-bit key. With a key
length of one, the maximum information the eavesdropper may obtain is
half of the message Alice sent.

“The second experiment was towards more practical schemes,
limiting Eve’s information to an
arbitrarily small amount using loga-rithm-length keys.”

Using free-space transmission rather than fiber allowed Lum’s team to
explore higher dimensions of encoding based on more complex combinations of quantum properties to allow
the transmission of error-correction
bits along with the message encoded
into the photon’s state. However, there
is a trade-off inherent in the use of error correction; the redundancy it introduces makes it easier for an adversary to
decrypt messages. As a result, a higher
key ratio is needed to guarantee security and successful communication over
noisy channels.

Liu and Lum both stress the experiments they performed were proof-of-concept demonstrations. Some of
the theoretical requirements for QDL
are not possible to realize in practice.
For example, the experiments by both
teams used the same technique as
that used for QKD to generate pairs of
photons. However, spontaneous parametric down-conversion is a random
process that can create more than two
daughter photons, with uncertain timing. Neither is desirable for QDL.

“Many reviewers pointed out that
our experiment was not stringent
enough to be considered truly secure;
we cannot guarantee that we limited